The present invention is directed to a device for measuring the distance of a surface of an object from a reference plane, which device has a light source producing a sharply focused light beam, a beam deflector which serves to deflect the beam and to repeatedly pass it over the surface of the object, a photo detector which responds only to light which reaches it along a given direction or sighting line and means for determining the distance of the surface from the reference plane in response to a signal from the detector.
A device for measuring the distance of a surface of an object from a reference plane which device utilizes a light source such as a laser for producing a sharply focused light beam, has a light deflector for deflecting the light beam along a given path, has a photo detector which will produce a signal when light reaches it along a sighting line of a given direction and has means for determining the distance based on the response from the signal detector is known and disclosed for example in German Offenlegungsschrift 25,08,836, which is an improvement of a measuring device disclosed in U.S. Pat. No. 3,954,335. In the device of the German Offenlegungsschrift, a beam divider is disposed in the path of the deflected beam to branch off partial light beams and two additional photo detectors, which will respond to a beam of light along a given direction, are provided. These additional photo detectors are arranged in such a way that they will receive the branch off sub-beams as the beam is deflected in two given angularly spaced positions. This device serves to stabilize the deflection speed of the light beam so that a permanently high degree of measuring accuracy can be achieved. For this purpose, the known device is operated in such a manner that the interval of time between a time at which one of the two additional photo detectors receives a branched or reflected sub-beam and the time at which the second of the two additional photo detectors receives a branched sub-beam is monitored. In the event that this time difference deviates from a theoretical value, the controls of the beam deflector are changed or regulated to cause the beam deflector to deflect the beam at a theoretical or desired speed.